1. Field of the Invention
The present invention relates generally to the control of liquid metal flow by electromagnetic means and, more particularly, is concerned with an electromagnetic flow control device incorporating a pumping action in controlling liquid metal flow.
2. Description of the Prior Art
The control of liquid metal flow by electromagnetic means has potentially many important industrial applications. One of these is in the area of vertical continuous casting of steel. FIG. 1 shows schematically a vertical continuous casting line, generally designated by the numeral 10. The continuous casting line 10 includes a ladle 12, a tundish 14, a casting mold 16, a spray zone 18 and a straightener 20 arranged in serial fashion. Hot molten metal M in a stream issues from the ladle 12 through a pouring tube 22 into the tundish 14. The tundish 14, in turn, infeeds the molten metal M contained therein in a free stream to an upper inlet end 24 of the casting mold 16. A continuous solid shell of metal S is formed around a liquid core in the casting mold 16 and withdrawn from a lower outlet end 26 thereof. The continuous shell or strand S is then fed through the spray zone 18 where it is progressively cooled to solidify the liquid core and thereafter the strand is fed between rolls 28 of the straightener 20.
In the case of liquid steel alloys, for example, there is a need to control the flow of the liquid metal M into the casting mold 16 in order to produce a uniform product. As schematically depicted in FIG. 2, one proposal is to provide an electromagnetic flow control valve 30 between the tundish 14 and the casting mold 16, preferably at an outlet nozzle 32 of the tundish 14. The valve 30 is needed at that location because the liquid steel level in the tundish 14 varies, creating variations in the pressure head which drives the flow. Also, temperature variations in the melt can adversely affect the quality of the cast product unless compensated for by varying the flow. Since there is little space available in this area, the valve 30 must be compact. It must also withstand the high temperatures involved (about 1500 degrees C. for liquid steel) and possible freezing and remelting of the liquid metal inside of the valve. Because it contains no moving parts and does not require close tolerance fittings, the electromagnetic valve 30 is considered very promising for this application.
The electromagnetic flow control valve 30, shown in schematic form in FIGS. 2-4, includes a single AC coil 34 disposed adjacent to the outlet nozzle 32 of the tundish 14 protruding from the bottom 36 thereof. Several versions of an electromagnetic valve and nozzle arrangement for controlling liquid metal flow rates or throughput are disclosed in U.S. patents to Garnier et al (U.S. Pat. Nos. 4,082,207 and 4,324,266) and also in an article entitled "Electromagnetic Devices For Molten Metal Confinement" by M. Garnier (edited by H. Branover, P. S. Lykoudes, and A. Yakhot, Vol. 24, Progress In Astronautics and Aeronatics, Inc., N.Y. (1983), pp. 433-441).
In FIGS. 4 and 5, the electromagnetic field quantities which govern its operation are indicated. The electromagnetic force density is given by (MKS units) EQU F=J.times.B (1)
Inside the valve 30, the magnetic field B is predominantly axially directed and the eddy currents J are azimuthally directed. (The eddy currents are represented by each of the respective pairs of circles with "." and "x" in them.) Therefore, by equation (1), which expresses the right hand rule, F is directed radially inward. This results in an effective pressure which squeezes on the liquid metal column C flowing through the nozzle 32. In the high frequency limit, this magnetic pressure, P.sub.m, is given by (MKS units) EQU P.sub.m =B.sub.o.sup.2 /4u.sub.o ( 2)
where B.sub.o is the peak amplitude of the (sinusoidally varying) magnetic field inside the coil 34 at the liquid metal surface and .mu..sub.o is the permeability of vacuum (liquid metals are non-ferromagnetic). At lower frequencies, this magnetic pressure is generally less than the equation (2) value.
An approximate analysis can be made of the valve operation, using Bernoulli's equation EQU P.sub.m +pgz+1/2pv.sup.2 =pgH (3)
where P.sub.m is non-zero primarily inside the valve 30 and where p=liquid metal density, g=acceleration of gravity, v=liquid metal velocity at height z relative to the valve exit, and H=the total liquid metal height measured from the valve exit. Therefore, at the valve exit, z=O, and solving (3) for v, one gets EQU v=.sqroot.2gH .sqroot.1-P.sub.m /pgH (4)
Without the magnetic pressure P.sub.m, the exit velocity, call it v.sub.o, would be given by EQU v.sub.o =.sqroot.2gH (5)
Therefore, the velocity ratio is EQU v/v.sub.o =.sqroot.1-P.sub.m /pgh (6)
and this is also the flow ratio, with and without a magnetic field, since the exit area is the same. Thus, the flow can be reduced when the magnetic field is present (P.sub.m &gt;O) according to equation (6).
Upon leaving the magnetic field region below the coil 34, P.sub.m becomes zero and the velocity must increase. Since the flow rate is now fixed at the valve exit value, the liquid column C must constrict. The above-cited patents and publication illustrate and describe the use of a metallic screen in order to achieve a sharp boundary between the magnetic field region and the field free region below the nozzle exit.
However, an electromagnetic flow control valve and a nozzle arrangement as described above can only control liquid metal flow through the nozzle by squeezing or constricting it radially inwardly. Since no net axially-directed force of any significance can be produced by the above-described valve arrangement, the arrangement cannot work as a pump in either upper or downward directions to respectively oppose or assist the liquid metal flow and cannot terminate the flow of metal. Consequently, a need still exists for a different design which will provide a pumping action.